Binary Number System
This number system is backbone of the digital electronics.
This number system has the minimum possible and useful base (radix) i.e. 2. This is because base0 system is not possible and base1 number system is not useful.
Every number in this system is denoted by using two symbols: 0 and 1.
The leftmost bit is called as Most Significant Bit (MSB) and rightmost bit is called as Least Significant Bit (LSB).
This number system has the minimum possible and useful base (radix) i.e. 2. This is because base0 system is not possible and base1 number system is not useful.
Every number in this system is denoted by using two symbols: 0 and 1.
The leftmost bit is called as Most Significant Bit (MSB) and rightmost bit is called as Least Significant Bit (LSB).
Symbols in any binary number are called BInary digiTS, or bits in general.
A group of 4bits is called a nibble.
A group of 8bits is called a byte.
And, a group of 16bits is generally called a word. (In fact, a word is said to group of any number of bits, that signify something; e.g. if a digital message has 12bits to convey info, word length is 12bit , as in 12bit ADC. But, generic notation is like 16bit: word, 32bit: doubleword etc.)
A group of 4bits is called a nibble.
A group of 8bits is called a byte.
And, a group of 16bits is generally called a word. (In fact, a word is said to group of any number of bits, that signify something; e.g. if a digital message has 12bits to convey info, word length is 12bit , as in 12bit ADC. But, generic notation is like 16bit: word, 32bit: doubleword etc.)
Complementary numbers
Like those for decimal numbers, the complement numbers do exist for binary numbers also. For binary number, 2's complement and 1's complement numbers exists.
(1's complement of N) = 2^n  N
(2's complement of N) = 2^n  N + 1
Instead of performing subtraction, 1's complement can also be found just by complementing the number bitbybit.
e.g. N = (1010)b = (10)d
so, n = 4
(1's complement of 1010) = 2^4  1010
= 1111  1010
= 0101
and we can see that 1010 and 0101 are two numbers complement to each other bitbybit.
( 2's complement of 1010) = (1's complement of 1010) + 1
= 0101 + 1 = 0111
(1's complement of N) = 2^n  N
(2's complement of N) = 2^n  N + 1
Instead of performing subtraction, 1's complement can also be found just by complementing the number bitbybit.
e.g. N = (1010)b = (10)d
so, n = 4
(1's complement of 1010) = 2^4  1010
= 1111  1010
= 0101
and we can see that 1010 and 0101 are two numbers complement to each other bitbybit.
( 2's complement of 1010) = (1's complement of 1010) + 1
= 0101 + 1 = 0111
Complements of binary numbers play an important role in arithmetic digital circuits like subtractor.

1's complement and 2's complement of any binary number exists . . . 

0 1 0 1 0 1 0 1 0 1